PHOTOSYSTEM-II EXCITATION PRESSURE AND DEVELOPMENT OF RESISTANCE TO PHOTOINHIBITION .1. LIGHT-HARVESTING COMPLEX-II ABUNDANCE AND ZEAXANTHIN CONTENT IN CHLORELLA-VULGARIS

The basis of the increased resistance to photoinhibition upon growth at low temperature was investigated. Photosystem II (PSII) excitation pressure was estimated in vivo as 1 - q(p) (photochemical quenching). We established that Chlorella vulgaris exposed to either 5 degrees C/150 mu mol m(-2) s(-1) or 27 degrees C/2200 mu mol m(-2) s(-1) experienced a high PSII excitation pressure of 0.70 to 0.75. In contrast, Chlorella exposed to either 27 degrees C/150 mu mol m(-2) s(-1) or 5 degrees C/20 mu mol m(-2) s(-1) experienced a low PSII excitation pressure of 0.10 to 0.20. Chlorella grown under either regime at high PSII excitation pressure exhibited: (a) 3-fold higher light-saturated rates of O-2 evolution; (b) the complete conversion of PSII alpha centers to PSII beta centers; (c) a 3-fold lower epoxidation state of the xanthophyll cycle intermediates; (d) a 2.4-fold higher ratio of chlorophyll a/b; and (e) a lower abundance of light-harvesting polypeptides than Chlorella grown at either regime at low PSII excitation pressure. In addition, cells grown at 5 degrees C/150 mu mol m(-2) s(-1) exhibited resistance to photoinhibition comparable to that of cells grown at 27 degrees C/2200 mu mol m(-2) s(-1) and were 3- to 4-fold more resistant to photoinhibition than cells grown at either regime at low excitation pressure. We conclude that increased resistance to photoinhibition upon growth at low temperature reflects photosynthetic adjustment to high excitation pressure, which results in an increased capacity for nonradiative dissipation of excess light through zeaxanthin coupled with a lower probability of light absorption due to reduced chlorophyll per cell and decreased abundance of light-harvesting polypeptides.